Sodium hydroxide and chlorine are important industrial starting materials. These are produced mainly by the electrolysis of sodium chloride. Processes for the electrolysis of sodium chloride have shifted from the mercury process, in which a mercury cathode is used, and the diaphragm process, in which an asbestos diaphragm and a soft-iron cathode are used, to the ion-exchange membrane process, in which an ion-exchange membrane as a diaphragm and an activated cathode having a low overvoltage are used. During the course of these developments, the electric power consumption rate for the production of 1 ton of caustic soda has decreased to 2,000 kWh.
Examples of processes for producing an activated cathode active in hydrogen generation for use in the ion-exchange membrane process include: a method in which a ruthenium oxide powder is dispersed into a nickel plating bath and composite plating is conducted onto an electrode base to obtain an active electrode; a method in which a nickel deposit containing a second ingredient such as sulfur or tin is formed by plating; and a method in which NiO plasma spraying is used. Examples thereof further include method in which Raney nickel, an Ni--Mo alloy, a Pt--Ru deposit formed by displacement plating, or the like is used. An activated cathode is also known in which a hydrogen-absorbing alloy is used in order to impart resistance to reverse current.
These techniques are described in the following publications (1) to (4).
(1) Electrochemical Hydrogen Technologies, pp. 15-62 (1990) PA1 (2) U.S. Pat. No. 4,801,368 PA1 (3) J. Electrochem. Soc., 137, pp. 1419-1423 (1993) PA1 (4) Modern Chlor-Alkali Technology, Vol. 3, pp. 250-262 (1986)
Recently, electrolytic cells which can be used in the ion-exchange membrane process at a heightened current density are being investigated in order to increase production capacity and reduce investment cost. Because low-resistance membranes have been developed, it has become possible to impose a high-density current load onto an electrode.
In the ion-exchange membrane process, the anode is usually an insoluble metal electrode (DSA). In view of the fact that DSA's have been used as anodes in the mercury process at current densities as high as up to 200 to 300 A/dm.sup.2, use of a DSA in electrolysis by the ion-exchange membrane process at such a high current density seems to pose no problem with respect to the anode alone. However, it is still unknown that existing cathodes can be used because their useful life and performance characteristics have not been confirmed at such high current densities in real cells.
Specifically, the cathode for use in the ion-exchange membrane process needs to exhibit the following characteristics: a low overvoltage; no damage to the membrane even upon contact with the cathode; and reduced release of fouling ingredients, e.g., metal ions. If there is no cathode which has these properties, a conventionally used cathode (one having high surface roughness and a catalyst layer of low mechanical strength) is employed. Basically, however, certain measures are necessary for the use of such a conventional cathode. On the other hand, for realizing the new process in which electrolysis is conducted at a high current density, there is a need to develop an activated cathode which has the above characteristics and is sufficiently stable even under the above-described electrolysis conditions.
FIG. 2 diagrammatically shows the currently most common process for sodium chloride electrolysis using an activated cathode. In this sodium chloride electrolysis, a cathode 3 is disposed on the cathode side, i.e., on one side, of a cation-exchange membrane 1 so that it is in contact with the membrane (zero gap) or is apart therefrom to form a gap of up to 3 mm. An anode 2 is disposed on the other side of the cation-exchange membrane 1. On the catalyst layer of the cathode 3, water containing sodium chloride reacts to yield sodium hydroxide.
The anode and cathode reactions are as follows. EQU 2Cl.sup.- .fwdarw.Cl.sub.2 +2e.sup.- (1.36 V) EQU 2H.sub.2 O+2e.sup.- .fwdarw.2OH.sup.- +H.sub.2 (-0.83 V)
The theoretical electrolytic potential is 2.19 V.
Conventional activated electrodes, when used in cell operation at a high current density, exhibit some serious problems which need to be solved as follows.
(1) Since the electrodes employ bases comprising nickel, iron, carbon, etc., these bases partly dissolve away as the electrodes deteriorate due to high current density. The dissolved base ingredients which have thus eluted into the catholyte move to the membrane and the anode chamber, leading to a decrease in product quality and impaired electrolytic performance.
(2) The overvoltage increases with increasing current density, resulting in reduced energy efficiency.
(3) As the current density becomes higher, the cell exhibits increased unevenness in the distribution of bubbles and in the concentration of the caustic soda that is produced. Hence, the catholyte exhibits an increased solution resistance loss.
It may be desirable to place the cathode 3 in contact with the ion-exchange membrane 1 such that there is no gap between the cathode material and the ion-exchange membrane, because this constitution should be effective in lowering the electrolytic voltage. However, because the cathode 3 has a rough surface, the cathode 3 may mechanically break the ion-exchange membrane 1 when used in contact therewith. Consequently, use of the conventional cathode 3 at a high current density in such a zero-gap constitution has been problematic.
If an existing cell needs almost no modification for efficient operation at both low and high current densities, this brings about a considerable economic advantage. On the other hand, when electrode deterioration has occurred, it is necessary to re-form the catalyst layer of the cathode. In many cases, however, this reactivation is technically or economically difficult.